Thermoelectric conversion process and apparatus



Aug. 13, 1968 R. FORMAN ETAL 3,397,327

THERMOELECTRIC CONVERSION PROCESS AND APPARATUS Original Filed March 20,1962 5 Sheets-Sheet 1 a 'z' I g :2 4 i 5 a .5 200mm Q 1 NATURAL u mwrrou5 1O 45 u moor: VOLTAGE (VOLTS) O 2 INVENTORS. 5 4o :0 SSIEIPH gS QFSILEY N A. moo: VOLTAGE (VOLTS) ROBERT L CUMMEROW ATTORNEY Aug. 13, 1968R. FORMAN ETAL THERMOELECTRIC CONVERSION PROCESS AND APPARATUS 5Sheets-Sheet 2 Original Filed March 20, 1962 Aug. 13, 19 8 R. FQRQANETAL 3,397,327

THERMOELECTRIC CONVERSION PROCESS AND APPARATUS Original Filed March 20,1962 5 Sheets-Sheet 5 Q "2 N a s: m 8 I 2% Q v Q q a Q 0T INVENTORS."

RALPH FORMAN 'Q 333 3 JOHN A. GHORMLEY ROBERT L. CUMMEROW ATTRNE Aug.13, 1968 R. FORMAN ETAL 3,397,327

THERMOELECTRIC CONVERSION PROCESS AND APPARATUS 5 Sheets-Sheet 4Original Filed March 20, 1962- INVENTORS. RALPH F'ORMAN JOHN A. GHORMLEYROBERT L. CUMMEROW Aug. 13, 1968 R. FORMAN ETAL THERMOELECTRICCONVERSION PROCESS AND APPARATUS 5 Sheets-Sheet 5 Original Filed March20, 1962 Q Q h w m w m L I I Q\\ o m u 2 o Him 5i w m on .LNHHHID HGONVINVENTORS'. RALPH FORMAN JOHN A.GHORMLEY ROBERT 1.. CUMMEROV Arm/w:

United States Patent 3,397,327 THERMOELECTRIC CONVERSION PROCESS ANDAPPARATUS Ralph Forman, Rocky River, Ohio, John A. Ghormley,

Oak Ridge, Tenn., and Robert L. Cummerow, Hartsdale, N.Y., assiguors toUnion Carbide Corporation, a corporation of New York Originalapplication Mar. 20, 1962, Ser. No. 182,707, now Patent No. 3,322,977,dated May 30, 1967. Divided and this application Aug. 5, 1966, Ser. No.570,575

6 Claims. (Cl. 3104) This application is a division of application Ser.No. 182,707 filed Mar. 20, 1962, now US. 3,322,977, which is in turn acontinuation-in-part of application Ser. No. 147,593, filed Oct. 25,1961, and now abandoned.

The present invention relates generally to a process and apparatus forconverting heat energy to electrical energy and, more particularly, to aprocess and apparatus for converting heat energy directly to electricalenergy by effecting thermionic emission from a hot body while producingions in the gas surrounding the hot body.

Heretofore, it has been proposed to convert heat energy to electricalenergy by using a gas having a very low ionization potential with a hotelectron-emitting material which has a work function higher than theionization potential of the gas. Such a process is employed in theconventional cesium thermionic converter, wherein heat energy isconverted directly to electrical energy by utilizing cesium gas, whichhas a very low ionization potential (3.8 ev.), in conjunction with a hottungsten cathode, which has a work function (4.6 ev.) higher than theionization potential of the cesium gas and thus effects ionization ofthe cesium gas. The general operating principle for the cesiumthermionic converter is that the ionized cesium produced by the hotfilament neutralizes the space charge which is ordinarily resp'onsibefor inhibiting themionic emission from the hot filament. Although theoperating principle of such a cesium thermionic converter is a soundone, effective electron emitters usually have low work functions, and arelatively small number of gases have such low ionization potentials.Thus, relatively few gases are suitable for use in such devices. Also,gases having low ionization potentials are often chemically active anddiffcult to contain in a closed system.

More recently, it has been found that cesium gas can be ionized, evenwhen the work function of the cathode is slightly below the ionizationpotential of the cesium, by employing high cathode temperatures (betweenabout 1500* and about 3000 C.) and maintaining the cesium at a pressurebet-ween about 0.1 and about 2.0 mm. of mercury. However, the pressureof cesium required in such a device necessitates operating at relativelyhigh ambient temperatures which, combined with the high chemicalactivity of cesium, makes construction of the device considerably moredifiicult. Also the cathode in such a device has a relatively shortlife.

It has also been found that the space charge surrounding a very hotcathode can be neutralized by surrounding the cathode with a rare gas.However, such a process usually requires very high cathode temperatures.It has also been found that cesium gas can be ionized at a relativelylow partial pressure in a rare gas, but this still requires the use ofchemically active cesium.

Another recent discovery is that fission recoil particles from auranium-bearing cathode can be used to ionize a noble gas in athermionic diode within a nuclear reactor. Such a device is described inthe Journal of Applied Physics, vol. 30, at p. 2017 (1959). However,some serious problems would be expected in such a device. For example,some of the fission products could act as cathode poisons. Also, theshort range of the fission recoil par- 3,397,327 Patented Aug. 13, 1968ticles requires that the fissionable material be essentially on thesurface of the cathode, thus restricting the choice of cathodematerials. Further, problems of mechanical weakness and volatility athigh temperatures would be expected in materials which are suitableelectron emitters and contain high concentrations of fissionable atoms.

It is, therefore, the main object of the present invention to provide athermoelectric conversion process and apparatus wherein the thermionicwork function of the cathode may be higher or lower than the ionizationpotential of the surrounding gas, a relatively low cathode temperaturemay be employed, and no fissionable material is required on the surfaceof the cathode.

It is another object of the invention to provide such a process whereinthe gas to be ionize-d is not necessarily chemically active.

It is a [further object of the invention to provide such a process andapparatus wherein the cathode has a relatively long life.

A still further object of the invention is to provide an improve-dprocess and apparatus for varying space charge effects near a hotcathode so as to vary the electron current obtainable therefrom.

Other aims and advantages of the invention will be apparent from thefollowing description and appended claims.

As used herein, the term C as applied to temperature figures over 800*refers to C brightness as measured by an optical pyrometer.

In the drawings:

FIG. 1 is a schematic diagram of experimental apparatus for carrying outthe inventive process;

FIG. 2 is a diagram of circuit for determining when the space charge hasbeen neutralized in the apparatus Olf FIG. 1;

FIG. 3 is a graph showing the anode current-anode voltagecharacteristics obtained with a vacuum and various pressures of naturalkrypton gas in the apparatus of FIG. 1 with the work function of theanode greater than the work function of the cathode;

FIG. 4 is a graph showing the anode current-anode voltagecharacteristics obtained with various pressures of fission-productkrypton in the apparatus of FIG. 1 with the work function of the anodegreater than the work function of the cathode;

FIG. 5 is a graph showing the anode current-anode voltagecharacteristics obtained at various filament or cathode temperatureswith fission-product krypton in the apparatus of FIG. 1 at a pressure of40* mm. with the work function of the anode greater than the workfunction of the cathode;

FIG. 6 is a graph showing the anode current-anode voltagecharacteristics obtained at various filament or cathode temperatureswith fission-product krypton in the apparatus of FIG. 1 at a pressure of120 mm. with the work function of the anode less than the work functionof the cathode;

FIG. 7 is an elevation view in cross-section of a preferred embodimentof the inventive apparatus for carrying out the inventive process; and

FIG. 8 is a graph showing the anode current-anode voltagecharacteristics obtained at various dose rates of ionizing electrons(produced by radiation from a cobalt-60 source) in natural krypton at apressure of mm. in the apparatus of FIG. 1..

In accordance with the present invention, there is provided a thermionicconverter comprising a cathode and an anode disposed in an ionizablegas, the cathode having a thermionic work function greater than thethermionic work function of the anode and being electrically connectedto the anode through an external load circuit, the

temperature of the cathode being sufficiently high to effect thermionicemission therefrom and the temperature of the anode being below thetemperature of the cathode and sufliciently low that the thermionicemission from the anode is negligible in comparison with the thermionicemission from the cathode; and a source of ionizing radiation forirradiating the ionizable gas with at least one type of chargedparticles selected from the group consisting of beta particles, protons,deuterons, tritons, alpha particles, and high energy electrons, thepressure of the ionizable gas and the dose rate of the ionizingradiation being sufficient to produce an ion concentration suflicientlyhigh to make the current output of the converter temperature dependent.

The ionizing radiation employed in the present invention may be producedby any convenient process. For example, beta particles may be obtainedfrom beta decay of a radioactive nuclide such as krypton-85, and highenergy electrons may be obtained from gamma radiation as a result of thephotoelectric process, Compton scattering, or pair production.High-energy protons or deuterons may be produced as a result ofcollisions of fast neutrons in a nuclear reactor with hydrogen ordeuterium. High-energy protons and tritons may be produced in a nuclearreactor by the absorption of slow neutrons in a material having nucleiwith a high cross section for an n, p or n, alpha reaction. Alphaparticles may be obtained from alpha decay of radioactive nuclides suchas radon.

The rate of formation of gas ions in the space between the cathode andanode is determined mainly by the pressure and type of the ionizable gasaround the cathode and anode, and the dose rate of the ionizingradiation, i.e., the energy, type, and flux of ionizing particlesemployed. The concentration of ions is also dependent on the rate ofrecombination. By varying these factors, the ion concentration in thegas around the cathode can be increased to the level required to makethe current output of the converter temperature dependent, and a cathodeoperating at a relatively low temperature can be employed. In general,the ion concentration increases with increasing gas pressure, increasingdose rate, and decreasing rate of recombination. When the range of theionizing particles exceeds the dimensions of the vessel, the ionconcentration is somewhat dependent on the geometry of the vessel.

After the ion concentration has been increased sufficiently to cause thecurrent output of the converter to be temperature dependent, the outputcan be increased even further by continuing to increase the dose rate ofthe ionizing radiation and/ or the pressure of the ionizable gas. Thecurrent obtainable by the present process in a given device is higherthan the current obtainable with the vacuum in the same device. However,it is generally preferred to have the source of ionizing radiation inthe form of a gas between the cathode and the anode. The radiating gasmay itself be the ionizable gas, or it may be mixed with other ionizablegases. Also, more than one type of radioactive gas may be employed.

One source of ionizing radiation suitable for use in the presentinvention is a source of slow neutrons, such as a nuclear reactor, incombination with a material having nuclei with a high cross section foran n, p (neutron in, proton out) reaction or n, alpha (neutron in, alphaparticle out) reaction. Examples of such materials are boron-l0,lithium-6, and helium-3. The boron and lithium are solids and may bedisposed within the ionizable gas in the diode in the form of a coatingon the anode or on the inner walls of the diode container. Such coatingsmay be formed, for example, by electroplating. The boronor lithium-6need not be used in elemental form, out may be contained in a suitablecompound, such as TiB Helium-3 is a gas and may be mixed with theionizable gas, preferably in an amount such that the resulting gasmixture contains less than about 10% by volume helium-3. Absorption ofslow neutrons from the reactor or other neutron source in helium-3, forexample, produces high-energy protons and tritons with kinetic energiesof about 0.6 mev. and 0.2 mev., respectively. In a reactor having a slowneutron flux of 10 neutrons/cm. -sec, pure helium-3 at a pressure of oneatmosphere in a container having a radius greater than the 6 cm. rangeof the protons would be subjected to a dose rate of 4 10 rads per hour(in the center of the container) from the products of the n, p reaction.With identical conditions in a vessel having a radius of one centimeter,the dose rate would be about 10 rads per hour and the rate of ionformation would be 10 ions/cc.-sec. A dose rate of 0.1 to 10,000megarads per hour is usually sufficient to make the current outputtemperature dependent. It is preferred to use a rare gas, such askrypton, as the ionizable gas, and the preferred pressure range for theionizable gas is from about 0.1 to about 200 millimeters of mercury.Although helium-3 is referred to herein as a source of ionizingradiation, it is to be understood that helium-3 becomes a source ofionizing radiation only when used with a neutron flux from a nuclearreactor to give the n, p reaction.

Another source of ionizing radiation suitable for use in the presentinvention is fission-product krypton. As used herein, the termfission-product krypton refers to a gas containing about 5% by volumekryptonand about by volume stable fission-product krypton isotopes whenfresh. Of course, as the fission-product krypton becomes older, theproportion of krypton-85 therein slowly decreases. The krypton-85 decaysto rubidium-85, which is a stable isotope of rubidium. In cases wherethe anode temperature is sufliciently low to permit the rubidium-85 todeposit thereon without being driven off, the rubidium-85 may be used tolower the work function of the anode. The fission-product krypton servesboth as the ionizable gas and as the source of ionizing radiation (betaparticles). Fission-product krypton is a relatively abundant and easilyisolated fission product having a specific activity of 21 curies pergram when fresh. Krypton-85 is a nearly pure (99.4%) beta emitter with ahalf-life of 10.5 years. When fresh fission-product krypton is employedin the present invention, a gas pressure of at least 10 mm. of mercuryis usually required to produce a concentration of gas ions sufiicient toreduce the space charge around a cathode (in the center of a vesselhaving a radius greater than the range of the beta particles)sufliciently to cause the current output of the diode to be temperaturedependent.

Another source of ionizing radiation suitable for use in the presentinvention is radon, which is an alphaemitting gas. About 1.0 rnillicurieof radon 222 and its short-lived decay products in natural krypton at apressure of about 20 mm. of mercury produces a concentration of gas ionssufficient to reduce the space charge around a cathode (in the center ofa vessel having a radius greater than the range of the alpha particles),sufficiently to make the current output of the diode temperaturedependent.

Still another source of ionizing radiation is a source of gamma rays,such as cobalt-60 or a nuclear reactor, in combination with a diodefilled with a rare gas. Absorption of gamma rays from the cobalt-60 orreactor in the walls (such as glass) and electrodes of the diodeproduces high-energy electrons which, in turn, ionize the rare gaswithin the diode. In this embodiment, the gamma-ray source may belocated completely outside the gas to be ionized. It is preferred tohave the dose rate from the high-energy electrons at least as great as0.05 megarad per hour. There is apparently no upper limit for the doserate, but as a practical matter there is no need to increase the doserate beyond the level which produces the maximum current which theelectrodes can carry. The krypton or other rare gas within the diodeshould be at a pressure of 1 to 200 millimeters of mercury. Theefficiency of the device is generally higher at the higher dose rates.

Any suitable electron-emitting material may be employed as the cathodein the present invention, regardless of whether its work function isgreater than, equal to, or less than the ionization potential of theparticular ionizable gas or gases employed. Typical examples of suitablecathode material are thoriated tungsten, which has a work function of2.6 ev., and porous tungsten containing embedded barium aluminate, whichhas a work function of 2.12 ev. These two materials are excellentemitters and can operate at relatively low temperatures. In order toobtain an output voltage from the inventive converter, the cathode musthave a thermionic work function greater than that of the anode and mustbe electrically connected to the anode through an external load. Wheneither of the two cathtode materials mentioned above is employed, theanode material may be an oxide cathode material (e.g., nickel coatedwith porous barium oxide-strontium oxide, which has a work function ofabout 1.0 ev.). Other suitable anode materials are nickel or tungstencoated with cesium or rubidium. The anode temperature must becontinuously maintained below the temperature of the cathode andsufficiently low that the thermionic emission from the anode isnegligible in comparison with the thermionic emission from the cathode;the relative temperatures of the cathode and anode are preferably suchthat the thermionic emission from the anode is less than about 0.1% ofthe emission from the cathode. The only requirement on the cathodetemperature is that it be sufficiently high to achieve efiectivethermionic emission therefrom. Although the spacing between the cathodeand anode is not critical to the operability of the present invention,the efficiency of the process may be varied to some degree by varyingthe spacing.

In addition to the ions produced by ionizing radiation, there may besome gas ions produced by thermal ionization.

An experimental embodiment of the inventive process and apparatus willnow be described by referring to the drawings.

A schematic view of the experimental apparatus is shown in FIG. 1. Aradioactive gas is contained at the required pressure in a Pyrex glassenvelope A cathode filament 12 is disposed within the gas and issupported at the top by a tantalum spring 14 and electrically conductivelead 20 and at the bottom by an electrically conductive lead 16. Thecathode 12 is heated to the temperature required to achieve effectivethermionic emission therefrom by an electrical current passed throughleads 16 and 20 from an external power source (not shown). An anode 18surrounds the cathode filament 12 in the form of a cylindrical sleeve.Lead 22 from the cathode and lead 24 from the anode connect thegenerator to an external load circuit 33.

In order to deter-mine the exact gas pressure and dose rate required tomake the current output of the device of FIG. 1 temperature dependent,the device is placed in the circuit shown in FIG. 2. Referring to FIG.2, V is the voltage required to heat the cathode 12 and is connectedacross the cathode through a resistance R with the polarity shown. Avariable voltage V is then connected across the anode 18 through a loadresistance R, and the resulting current I is measured as V is varied. Byplotting the I V characteristics at increasing gas pressures and/or doserates, it can be determined at what pressure and dose rate thecharacteristic becomes temperature dependent. The current output maythen be turther increased by continuing to increase the gas pressureand/or the dose rate. An oscilloscope S may be connected across the loadfor use in determining the required impedance of the load.

In an example of the aforedescribed process for de-' termining the gaspressure and dose rate required to make the output temperaturedependent, a tube such as that shown in FIG. 1 was prepared using athoriated tungsten cathode filament and a tantalum anode sleeve. The

tube was about 6 inches long and about 40 mm. in diameter with theelectrodes located in the center of the tube. The tube was placed in thecircuit shown in FIG. 2 and the anode current-anode voltagecharacteristic was taken both in a vacuum and in various pressures ofnatural krypton. The temperature of the cathode was about 1550" C.,while the temperature of the anode was less than 500 C. The spacingbetween the cathode and anode was about 0.5 cm. The curves obtained areshown in FIG. 3. The ordinate in FIG. 3 is the current I indicated inthe circuit shown in FIG. 2, and the abscissa is the voltage V indicatedin FIG. 2. Each of the curves obtained with the vacuum and the naturalkrypton was independent of filament temperature (space charge limited)over a range of 1550 to 1750 C.

The tube was then evacuated, filled with radioactive fission-productkrypton, and placed in the same circuit. The cathode and anodetemperatures and the spacing between the cathode and anode were the sameas when the vacuum and ordinary krypton were employed. Anodecurrent-anode voltage characteristics were again taken at variouspressures and various temperatures at each pressure; the gas pressurewas varied by varying the temperature around a cold finger, such as 9 inFIG. 1. The anode current-anode voltage curves obtained for thefission-product krypton are shown in FIG. 4. It can be seen from thecurves that the current output increased with increasing pressures offission-product krypton gas. Moreover, at pressures of 20 mm. and above,the curves became temperature dependent. The temperature dependence ofthe I V curve at a pressure of 40 mm. is shown in FIG. 5 over the rangeof 1550 C. to 1750 C. this shows that the inhibiting space chargesurrounding the cathode was elfectively neutralized by the inventiveprocess; the beta emission from the Kr at pressures of 20 mm. and aboveincreased the ion concentration sufiiciently to at least partiallyneutralize the space charge, thereby permitting more electrons to flow.The 2 mm. curve in FIG. 4 was still temperature independent because theion concentration at that pressure did not sufiiciently neutralize thespace charge. At the pressure of 40 mm. of mercury, there were about twocuries of Kr in the tube, and the dose rate was about 0.10 megarad perhour; at a pressure of 20 mm. of mercury, there was about one curie ofKr in the tube, and the dose rate was about 0.025 megarad per hour. Nooutput current was produced at V =0 because the tantalum anode had awork function (4.0 ev.) greater than that of the thoriated tungstencathode (2.6 ev.).

After it had been determined that the fission-product krypton at apressure of at least 20 mm. of mercury would make the current outputtemperature dependent, the tantalum anode was replaced with an anode ofoxide cathode material. Since the oxide cathode material had a workfunction (1.0 ev.) well below that of the cathode (2.6 ev.), as opposedto the 4.0 ev. work function of the tantalum anode, the device could nowproduce an output current in the absence of an applied voltage, i.e., atV =0. In order to illustrate the operation of the device as a generator,it was again placed in the circuit of FIG. 2 and the I -V characteristictaken at various pressures and temperatures. The curves obtained withthe fission-product krypton gas at a pressure of mm. of mercury over atemperature range of 1550 to 1750 C. are shown in FIG. 6. It can be seenfrom the curves of FIG. 6 that a substantial current was produced at V=0. Also, the output was temperature dependent, which shows that thespace charge surrounding the cathode was substantially neutralized.Results similar to those described above for the thoriated tungstencathode and tantalum anode were obtained with an oxide cathode filamentand a tantalum anode.

In another example of the invention, a tube similar to that shown inFIG. 1 was prepared with a thoriated tungsten cathode (2.6 ev.) and atantalum anode (4.0 ev.).

The tube was filled with natural krypton gas at a pressure of 80 mm. ofmercury and subjected to gamma rays from cobalt-60. The tube was thenplaced in the circuit shown in FIG. 2, and anode current-anode voltagecharacteristics were taken at various dose rates. The temperature of thecathode was about 1650 C., while the temperature of the anode was lessthan 400 C. The spacing between the cathode and anode was about 0.5 cm.The curves obtained are shown in FIG. 8. It can be seen from the curvesthat the current output increased with increasing dose rates. At doserates up to 0.5 megarad/hr., the maximum current varied almost directlyin proportion to the dose rate; between 0.5 and 1.0 megarad/hr., theincrease in current was less than proportional to the increase in doserate, indicating that the current was becoming plasma-limited.

Another form of the inventive apparatus is shown in FIG. 7. Thisembodiment comprises a pair of concentric conductive cylinders 51 and 52held in place by insulating end rings 58 and 58' so as to define anannular chamber 53. Aflixed to the outer surface of the inner cathodesupport cylinder 51 is a cathode sleeve 50 of tungsten impregnated withbarium aluminate (work function of 2.12 ev.). The chamber 53 isexhausted through tube 55 and then filled through the same tube withradioactive fission-product krypton at a pressure of at least mm. ofmercury. The cathode 50 is connected to an external load circuit throughconductor 59 while the anode sleeve 61 (oxide cathode material onnickel) is connected to the same load circuit through conductor 60. Thecathode 50 is heated by passing an appropriate hot fuel or gas throughthe tubular passageway 57. In order to maintain the anode 61 at atemperature below that of the cathode 50, the ambient temperatureoutside the outer cylinder 52 is maintained at a temperature below thatof the hot gas in the passageway 57.

Alternatively, the chamber 53 could be filled with helium-3 mixed with arare gas, and the entire apparatus placed in a reactor. In such a case,the cylinder 51 could be heated by a uranium fuel element. Similarly,the chamber 53 could be filled with a rare gas and a radioactive cathodeor anode material employed. In these cases, the same procedure outlinedabove could be used to determine the gas pressure and dose rate requiredto make the current output temperature dependent.

While various specific forms of the present invention have beenillustrated and described herein, it is not intended to limit theinvention to any of the details herein shown, but only as set forth inthe appended claims.

What is claimed is:

1. A thermionic converter comprising a cathode and an anode disposed inan ionizable gas, said cathode having a thermionic work function greaterthan the thermionic work function of said anode and being electricallyconnected to said anode through an external load circuit, the

temperature of said cathode being sufiiciently high to effect thermionicemission therefrom and the temperature of said anode being below thetemperature of said cathode and suificiently low that the thermionicemission from said anode is negligible in comparison with the thermionicemission from said cathode; a material having nuclei with a high crosssection for an n, p or n, alpha reaction disposed within said ionizablegas; and means for irradiating said material with slow neutrons so as toproduce charged particles which ionize said ionizable gas, the pressureof said gas and the dose rate of said charged particles being sufficientto produce an ion concentration sufiiciently high to make the currentoutput of said converter temperature dependent.

2. The thermionic converter of claim 1 wherein said material disposedwithin said ionizable gas is at least one material selected from thegroup consisting of boron-10, lithium-6, and helium-3.

3. The thermionic converter of claim 1 wherein said means forirradiating said material with slow neutrons is a nuclear reactor.

4. The thermionic converter of claim 1 wherein said ionizable gas is arare gas.

5. The thermionic converter of claim 1 wherein the pressure of saidionizable gas is between about 0.1 and about 200 millimeters of mercuryand the dose rate of said charged particles is between about 0.1 andabout 10,000 megarads per hour.

6. A process for thermionic conversion comprising disposing a cathodeand an anode in an ionizable gas, said cathode having a thermionic workfunction greater than the thermionic work function of the anode andbeing electrically connected to said anode through an external loadcircuit, the temperature of said cathode being sufiiciently high toeffect thermionic emission therefrom and the temperature of said anodebeing below the temperature of said cathode and sufficiently low thatthe thermionic emission from said anode is negligible in comparison withthe thermionic emission from said cathode; disposing within saidionizable gas a material having nuclei with a high cross section for ann, p or n, alpha reaction; and irradiating said material with slowneutrons so as to produce charged particles which ionize said ionizablegas, the pressure of said gas and the dose rate of said chargedparticles being suflicient to produce an ion concentration sufiicientlyhigh to make the current output of said converter temperature dependent.

MILTON O. HIRSHFIELD, Primary Examiner.

D. F. DUGGAN, Assistant Examiner.

1. A THERMIONIC CONVERTER COMPRISING A CATHODE AND AN ANODE DISPOSED INAN IONIZABLE GAS, SAID CATHODE HAVING A THERMIONIC WORK FUNCTION GREATERTHAN THE THERMIONIC WORK FUNCTION OF SAID ANODE AND BEING ELECTRICALLYCONNECTED TO SAID ANODE THROUGH AN EXTERNAL LOAD CIRCUIT, THETEMPERATURE OF SAID CATHODE BEING SUFFICIENTLY HIGH TO EFFECT THERMIONICEMISSION THEREFROM AND THE TEMPERATURE OF SAID ANODE BEING BELOW THETEMPERATURE OF SAID CATHODE AND SUFFICIENTLY LOW THAT THE THERMIONICEMISSION FROM SAID ANODE IS NEGLIGIBLE IN COMPARISON WITH THE THERMIONICEMISSION FROM SAID CATHODE; A MATERIAL HAVING MNUCLEI WITH A HIGH CROSSSECTION FOR AN N, P OR N, ALPHA REACTION DISPOSED WITHIN SAID IONIZABLEGAS; AND MEANS FOR IRRADIATING SAID MATERIAL WITH SLOW NEUTRONS SO AS TOPRODUCE CHARGED PARTICLES WHICH IONIZE SAID IONIZABLE GAS, THE PRESSUREOF SAID GAS AND THE DOSE RATE OF SAID CHARGED PARTICLES BEING SUFFICIENTTO PRODUCE AN ION CONCENTRATION SUFFICIENTLY HIGH TO MAKE THE CURRENTOUTPUT OF SAID CONVERTER TEMPERATURE DEPENEDENT.